The invention is directed to an optical filter which is tunable, i.e. adjustable, with regard to the center wavelength characterizing the filters spectral response. In particular, the subject filter is preferably tuned by either optical or electrical means.
Optical filters have become essential components in wavelength multiplexed communications systems and in systems which use optical amplifiers. Such filters are used as elements to add or drop a selected wavelength and so may be used to block broadband noise while passing a signal, to flatten optical amplifier gain, or to direct a signal of particular wavelength into pre-selected nodes. Greater system flexibility can be achieved using filters whose wavelength response is tunable over a range of wavelengths.
A number of alternative filtering devices are known including, Fabrey-Perot or Mach-Zehnder interferometers, multiple layer dielectric film filters, and filters based on waveguide Bragg or long period gratings. Tuning of these devices may be accomplished by means that change the device refractive index or dimensions. For example straining a device, by bending or stretching a device or a portion thereof, can serve to alter both dimensions and refractive index. In a similar way, dimensions or index may be altered by altering the temperature of a device or a portion thereof. Thermo-electric cooling and heating is a convenient way to carry out thermal adjustment of a device. In addition, optical or electrical means can be used to alter device dimensions or refractive index and thus the device filtering characteristics. These latter means are usually preferred because they provide a filter having a faster response, and which is more reliable, and afford more reproducible control of the device, as compared to devices tuned by mechanical or thermal means.
There is therefore a need in the art for a tunable filter device having:
rapid response to a tuning means;
a high degree of reliability; and,
a high degree of reproducibility.
An optical fiber grating is a periodically or quasi-periodically perturbed waveguide for electromagnetic radiation, the grating, i.e., the perturbation having a preselected length along which the refractive index or the profile of the waveguide changes periodically.
The period of a grating is the distance between corresponding points in two nearest neighbor high or low refractive index portions of the grating.
A long period grating is one that provides a resonance between cladding modes and a core mode propagating in the same direction.
A structural resonance occurs when electromagnetic waves, such as light, bounce around within a structure because of total or near-total internal reflection from a boundary between a high and a low index region and comes back on itself in phase after a single or multiple reflections. Fabrey-Perot interferometers are the simplest example of one dimensional structural resonance. For structural resonance to occur in a waveguide in the transverse plane, the waveguide must be surrounded by a medium of lower refractive index than that of the waveguide. In a circular waveguide, such as an optical fiber, structural resonance occurs within the cladding region because of the total internal reflection at the clad-air or clad-jacket interface. In the case of optical fiber, light incident substantially normal to the usual direction of propagation gets totally internally reflected by the clad-air or clad-jacket interface and at certain wavelengths after many such reflections comes back on itself in phase to constructively interfere and hence cause a structural resonance. (A good reference for this is: S. C. Hill and R. E. Benner, xe2x80x9cMorphology Dependent Resonancesxe2x80x9d, in P. W. Barber R. K. Chang eds. xe2x80x9cOptical Effects Associated With Small Particlesxe2x80x9d, World Scientific (New Jersey. 1988)). FIG. 5 shows an example of structural resonance that can occur in an additional layer surrounding and in contact with the cladding layer of an optical waveguide. In this example, a laser is used to direct light into the layer in a direction substantially perpendicular to the layer surface, The structural resonance of the incident light which occurs in the layer changes the intensity dependent term of the refractive Index of the layer and so changes the peak wavelength filtered by an associated grating.
A Bragg grating is one which produces a resonance between a core mode and a counter-propagating, reflected core mode.
Throughout this document the term waveguide is taken to mean single mode waveguide unless expressly stated otherwise.
The tunable filtering device of this application meets the need for high performance tunable filters by providing an optically or electrically controlled long period or Bragg grating device.
A first aspect of the invention is a tunable optical filter which includes a single mode optical waveguide having a grating impressed upon at least a portion of the waveguide core. The tunability derives from an additional layer applied to the outer surface of the waveguide clad layer. This additional layer is made of a material whose refractive index may be changed by a control mechanism which acts upon the additional layer. Changing the refractive index of this outermost layer, changes the boundary conditions of the electromagnetic fields propagated in the waveguide. This change in boundary conditions will affect the propagation constant of the cladding modes. Depending upon the distance of the additional layer from the core-clad boundary, the change in refractive index of the additional layer may also affect the propagation constant of the core mode. For a typical single mode waveguide this distance is in the range of about 5 xcexcm to 10 xcexcm. The resonance wavelength of the grating depends directly upon the propagation constants of the resonating modes; Thus, changing the propagation constant effectively changes the resonance wavelength of the grating, effectively tuning the resonant peaks of the grating.
An embodiment of the tunable filter has an additional layer which is electro-optic, for example LiNbO3. The refractive index of the layer can then be changed rapidly and reproducibly by means of a voltage applied across the layer. The applied voltage effectively changes the propagation constant of the cladding mode and thus changes the resonant wavelength peaks of the grating. This is one embodiment of the long period grating.
In a preferred embodiment of this aspect of the invention, a structural resonance is established in the additional layer by directing light from one or more light sources onto the layer, the direction of travel of the incident light being transverse to the long dimension of the layer. At structural resonance, light intensity becomes more concentrated in the layer. The light intensity changes the intensity dependent term of the refractive index of the layer and so changes the propagation constant of a cladding mode. The intensity dependent term is commonly called the non-linear refractive index term. One writes the refractive index as n=n1+n2l, in which n1 is the linear index, l is light intensity and n2 is the nonlinear index coefficient. The grating is effectively tuned from one wavelength peak to another by controlling the incident light intensity. A typical light source is one or more lasers which direct light into the additional layer in a direction transverse to the long dimension of the layer.
As the non-linearity coefficient n2 of the material of the additional layer increases, the structural resonance induced index change in the additional layer is greater, so that the effect of the change in the additional layer on the propagating modes, either cladding or core, becomes greater. A typical non-linearity coefficient of a dispersion shifted waveguide is about 2.3xc3x9710xe2x88x9220 m2/W.
The effectiveness of the additional layer, as measured by the width of the tuning band, is expected to be enhanced in layer materials having a relatively higher nonlinear coefficient. The inventors contemplate coefficients on the order of at least 10-19 at this time. Profiles designed to increase non-linear index coefficient are under study for example in co-pending provisional application No. 60/071732 incorporated herein by reference. A typical tuning band width is in the range of about 70 xcexcm. Thus, a preferred embodiment of an additional layer, in which structural resonance is to be established, is an additional layer comprising a material having a non-linearity coefficient in the range of about 10xe2x88x9220  10xe2x88x9219 m2/W.
In yet another embodiment of the novel tunable filter, the additional layer comprises a dye doped silica glass. The refractive index of such a dye doped glass may be changed by launching light transversely into the glass, thereby tuning the wavelength of the filter.
To avoid interaction of the transversely launched light with the signal light propagating in the waveguide, the wavelength of light, used to change the refractive index of the additional layer by means of structural resonance or interaction with a dye, is preferably outside the range of about 1300 nm to 1700 nm, which is an operating band of optical communication systems.
In yet another embodiment of the tunable filter, the additional layer comprises a piezoelectric material, for example the material may be a soft polymer. The density of the material, and thus the refractive index of the material, can be changed by applying a voltage across the material, thereby tuning the grating to a different resonance wavelength.
In an embodiment of this first aspect of the novel tunable filter, In which the boundary of additional layer is sufficiently close to the mode propagating in the core to change the propagation constant thereof, as is noted above, the grating period may be chosen to be that of a Bragg grating,
In a second aspect of the invention, the waveguide, having a core and a clad and an additional outermost layer, contains a grating of period xcex9g which is chosen such that the difference in the propagation constant of a cladding mode, xcex2cl, and the propagation constant of a core mode, xcex2c, are related by the equation xcex2clxe2x88x92xcex2c=2xcfx80/xcex9g, the condition which defines resonance between the modes. Then the filter may be tuned by changing xcex2cl. The xcex2cl may be changed by changing the refractive index of the additional layer by any of the means noted above.
In a third aspect of the novel tunable filter, the grating constant may be chosen as xcex9b, a constant appropriate for Bragg grating. The resonance which is established is then governed by the equation, xcex2rxe2x88x92xcex2c=2xcfx80/xcex9b, where xcex2r is a reflected mode. In this aspect, the filter is tuned by changing xcex2c. Thus the thickness of the cladding layer much be chosen small enough to allow interaction between the core mode and the additional layer-cladding layer boundary. Then xcex2c may be changed by changing the refractive index of the additional layer by any of the means noted in the first or second aspect of the invention.